[ReprinteJ from the Joudnal oi- thr Fhanklix Ixstitutk, April, 187S.]

ON THE RELATION OF MOISTURE IN AIR TO HEALTH

AND COMFORT.

By RoBT, Briggs, C.E., Cor. Mem. Am. Inst, of Architects, etc.

c

In continuation of the paper read before the American Institute of
Architects,' which has appeared in the three previous numbers of this
Journal, it is desirable to support the argument for the impractica-
bility of attaining the full summer condition of humidity, for air in
•winter which has been warmed to the temperature of comfort, by
more extended computations than could be presented to an audience,
or brought within the scope of comprehension of the listener or
the casual reader. The single example with specified relations, which
was taken, showed, on calculation, that nearly as much heat would
be expended in supplying vapor for the usual hydration of air of
usual humidity and of 34° temperature (which air was heated to 70°),
as was expended in heating the air itself.

The temperatures, and humid condition of about 69 per cent,
assumed, being the averages of nature, out of doors, for three warm
or three cold months, in the city of Philadelphia, in 1844, as reported
by Prof. Bache. It occurs, accidentally, that the exact conditions of
temperature and humidity chosen for an example, give the ratio
of heat demanded for the two purposes incident to warming air as
unity, while this ratio, for other temperatures and degrees of humid-
ity, will be found to vary materially. For the purpose of exhibiting
more completely the general case, the accompanying table has been
prepared to show what quantities of heat are demanded for heating,
or are given out in cooling air of 70 per. cent, humidity, from various
temperatures (from 0° to 100°) to 70°, in direct comparison to the
quantities for vaporization or condensation of water to procure the
condition of 70 per cent, humidity to the air of 70°. When 70°
and 70 per cent, humidity may be accepted as the American summer
condition of comfort for the air; it being asserted that 70°, with 80
per cent., or more, of humidity, is close and enervating, while 70°,
with GO per cent., or less, of humidity, is fresh and cool, and de-
mands heavy clothing to preserve the comfort of the individual — a
proper and necessary demand in winter.

' At the Meeting of the Institute at Boston, October 18th, 1877.

2

Table.

Volume and Weights of Air and Vapor qv Humidity — Tempkeatures of .Satura-
tion AND Dew Point — Heat to be Expended in Warming or Cooi-ing Air
OR Vapor to the Temperature of 70° Fahrenheit and Condi-
tion OF 70 per cent. Humidity.

I.

II.

III.

IV.

V.

VI.

VII.

VIII.

IX.

X.

XI.

XII.

XIII.

XIV.

XV.

Temperatures in degrees' Fahrenheit.

Volume of dry Air to one Pound,
Barometer 29'92'2 Inches.

Weight of dry Air per 1000 cubic feet.
Barometer 29-922 inches.

Force of Vapor in inches of Mercury
Column.

Weight of Vapor in 1000 cubic feet =
quantity of Vapor in saturated Air.

70 per cent, of vreight of Vapor in 1000
cubic feet = quantity of Vapor in air
of 70 per cent, humidity.

Weight of 1000 cubic feet of Air of 70
percent, humidity, including its va-
por. Barometer, 29-922 inches.

Weight of dry Air in 1000 cubic feet
of Air of 70 per cent, humidity. Bar-
ometer, 29-922 inches.

Weight of Vapor accompanying 73-63
lbs. of dry air at given temp., iind 70
per cent, humidity. Barom., 29-922 in.

Percentage of volumes of Air of 70 per
ct. humidity displaced by the presence
of aqueous vaj)or. Barom., 29-922 in.

Temperatures of Saturation in degrees
Fahr. = Dew Point of Air of various
temperatures, and 70 per ct. liumid ity.

Temperature of indication of Wet Bulb
Thermometer, taken from Glaisher's
tables.

Weiglit of Vapor to be supplied or
taken from 1000 cu. ft. of Air, when
brought to 70° Fahr. to make quan-
tity needed for 70 per cent, humidity.

Heat units for warming orcool'g 0-07363
lbs. of Air with its accompanying
vapor from given temp, to 70° Fahr.

Heat units demanded for vaporization
or condeni. of vapor to give TO per ct.
humidity to one cu. ft. of Air of 70° F.

0

11-577

86-38

0-045

0-081

0-057

86-35

,86-29

0-048

0-11

0-749

1-228

0-797

5

11-704

85-44

0-054

0-095

0-067

85-40

85-33

0-068

0-13

0-739

1-141

0-787

10

11-830

84-53

0-067

0-118

0-082

84-48

84-40

0-072

0-16

1-1

8-8

0-725

1-054

0-772

15

11-957

83-64

0-085

0-148

0-104

83-58

83-47

0-092

0-20

7-3

13-9

0-705

0-966

0-751

20

12-083

82-76

0-108

0-186

0-130

82-68

82-55

0-116

0-25

12-4

18-9

0-681

0-879

0-725

25

12-210

81-90

0-136

0-232

0-162

81-80

81-64

0-146

0-32

17-2

23-5

0-651

0-792

0-693

30

12-336

81-06

0-169

0-285

0-199

80-94

80-74

0-182

0-40

21-8

27-7

0-615

0-704

0-055

32

12-387

80-73

0-183

0-308

0-216

80-60

80-38

0-198

0-43

23-6

28-8

0-599

0-670

0-638

35

12-463

80-24

0-207

0-345

0-242

80-10

79-85

0-223

0-49

26-5

25-

0-574

0-617

0-611

40

12-590

79-43

0-251

0-415

0-290

79-26

78-96

0-271

0-59

30-8

30-1

0-526

0-.529

0-560

45

12-716

78-64

0-302

0-494

0-346

78-43

78-08

0-326

0-71

35-5

34-6

0-471

0-432

0-501

50

12-843

77-87

0-362

0-587

0-411

77-62

77-21

0-392

0-86

40-3

45-

0-405

0-353

0-432

65

12-969

77-11

0-433

0-695

0-487

76-82

76-33

0-470

1-01

45-1

49-

0-328

0-266

0-349

60

13-096

76-36

0-517

0-821

0-575

76-02

75-44

0-561

1-21

50-0

54-

0-236

0-178

0-251

65

13-222

75-63

0-616

0-968

0-678

75-22

74-54

0-669

1-44

54-9

58-5

' 0-127

0-088

0-136

70

13-349

74-91

0-732

1-138

0-797

74-43

73-63

0-797

1-71

59-8

63-

0-000

0-000

0-000

75

13-476

74-21

0-867

1-336

0-935

73-65

72-71

0-946

2-03

64-7

68-

0-149

0-090

0-159

80

13-602

73-52

1-023

1-562

1-094

72-86

71-77

1-121

2-39

69-5

72-5

0-324

0-181

0-345

85

13-729

72-84

1-203

1-822

1-275

72-07

70-80

1-326

2-82

74-2

77-5

0-529

0-272

0-563

90

13-855

72-17

1-410

2-117

1-482

71-28

69-80

1-563

3-30

78-9

82-5

0-766

0-305

0-816

95

13-982

71-52

1-647

2-452

1-716

70-49

68-77

1-837

3-85

83-7

87-5

1-040

0-460

1-108

100

14-108

70-88

1-918

2-828

1-980

69-69

67-71

2-152

4-49

88-4

92-5 '

1-355

0-556

1-443

The figures relating to weights in the foregoing table, refer to 1000
cubic feet (a cube of 10 feet side) for a unit, to avoid the repetition
of O's in decimals; by multiplying any value by 7, the weights per cubic
foot in grains will be found. All the weights are in pounds avoir-
dupois and decimals. The volumes are in cubic feet.

Columns II and III are derived from Regnault's data ; weight of one
cubic foot of dry air = 0-080728 pound = 12-387 cubic feet per
pound; and rate of expansion = 0-002039 volume for each degree
Fahrenheit from 32° = 0-025315 cubic foot per pound from 32°.

Column IV is taken from Guyot's tables, Regnault's data, slightly
corrected by differences.

Column V is derived from W = 2Q.^-)o ^ Column III

X value of Column IV, where 0-622 = specific gravity of vapor in
air of the same tension ; and 29-922 = inches of mercury column
■which measures the tension of air in Column II.

Column VI is obtained directly from Column V, as indicated in
caption.

Column VII is obtained by taking the difference of weight of the
volume of vapor in the air, from that of the air which it displaces,
and deducting their difference from the respective values in Column
III. The values in Column VII agree as closely as could be expected
with results in Glaisher's tables.

Column VIII is obtained by deducting weight of vapor, Column
V from Column VII.

Column IX is obtained by taking the ratio of quantity of dry air
in the several values of Column IX, with the value at 70° (= 73-63
pounds) as a unit, and dividing by this ratio the values in Column VI.

F

Column X is obtained by taking 70 per cent, of 29 •92'^' where F

is the tension of vapor possible to exist at the given temperature, as
given in Column IV, and 29-922 is the tension of the atmosphere.
It should be noticed that this column has its units in percentages :
the first value, for instance, is -^^^q of a per cent.

Column XI is obtained by making a computation of three columns
for saturated air, like vil, Vlll and ix, and thus ascertaining at
what degree of temperature the values in Column IX coincide with
those in the same column for saturated air. This column closely
agrees with Glaisher's, varying two degrees higher at 15° and corres-
ponding at 80°.

Column XII is taken from Glaisher's tables at 70 per cent., and is
sufficiently exact for use.

Column XIII is obtained by taking the difference between the
quantity of vapor in air of 70° Fahrenheit, and that in the several
values given in Column IX.

4

Column XIV is obtained by multiplying 0-07363 pounds (the
quantity of dry air in a cubic foot of air of 70° Fahrenheit and 70
per cent, humidity) by 0-238 (the specific heat of air), adding to this
value, that of the amount of the several values in Column IX, multi-
plied by 0-475 (the specific heat of vapor), and then multiplying the
sums of the above by the number of degrees from 70° which the air
is to be heated or cooled.

Column XV is obtained by multiplying the values of Column IX
(the vapor to be supplied or removed) by 1065° = the latent heat of
steam from and at 70°, and dividing the result by 1000, so as to give
the heat of vaporization belonging to one cubic foot of air at 70°.

A careful study of. the table, with comparisons of the values given
in the several columns, will readily allow any student to find the
fullest corroboration of the views expressed in the original paper ;
but for the convenience of the general reader, it may be well to ex-
hibit some of the results. Thus, calling attention to Column XI, it
will be seen that the air of saturation at 60° becomes, by heating,
that of 70° and 70 per cent, humidity ; showing a close approach of
English and American humid conditions to the comfortable condition
in either case. Again, the temperature of saturation for air of 80°,
with 70 per cent, humidity, is 69-5° ; consequently, air of above 80°
of the usual humid condition of our climate in summer, will become a
mist and rain, unless its moisture be absorbed by some non-heat-giving
absorbent. If such air is to be reduced to the 70° standard of com-
fort, by means of surfaces cooled by currents of water, such surfaces
must have a temperature at least below 60°, and probably 10° below
that point, to abstract, by condensation, the moisture from the air.
The heat demanded for condensation, therefore, will be increased
some 20° below that of cooling the air, but as the latent heat of vapor
of water is nearly constant, the difference of quantity of heat for
cooling is not materially changed.

A study of the last two columns will demonstrate that as the quan-
tity of moisture present in air falls ofi" much more rapidly than the
temperature, the quantity which is requisite to make up the moisture
in air at 70° and 70 per cent, humidity, from the condition at the
various temperatures with the same ratio of humidity, becomes more
and more neai'ly the same as the temperatures fall ofi". The great heat
demanded for changing water into vapor accordingly, is nearly con-
stant for the low temperatures, and the heat demanded for warming

5

air gains on tliat for vaporization of water ; and after passing a greatest
difference at between 55° and 60°, attaining an equality at about 36°,
it becomes about one-half greater at 0°.

The tabular values here given have, it is thought, been first grouped
together in this, or any form, and they will be found a ready and con-
venient basis for the application of the various meteorological condi-
tions to warming and ventilation.